The present invention relates to a circuit arrangement with at least two semiconductor switches and with an overvoltage protection for the at least two semiconductor switches.
Semiconductor switches such as MOSFETs or IGBTs are being used increasingly for switching electrical loads, such as switching electrical consumers in motor vehicles. In order to be able to actuate several consumers by means of a single integrated circuit, several semiconductor switches, in particular several power MOSFETs, are integrated in a semiconductor chip. An integrated circuit with two power MOSFETs integrated in a chip is, for example, the integrated circuit HITFET@ BTS 3410 G, which is described in data sheet 2004-03-05 of Infineon Technologies AG, Munich.
Such an integrated circuit with several semiconductor switches has circuits available separately for each of the semiconductor switches. Such circuits are, for example, driver circuits, one of which is assigned to each semiconductor switch, or protection circuits, such as current limiting circuits, overheating circuits, overvoltage protection circuits, or circuits to prevent overloading, one of each being assigned to each semiconductor switch.
The overvoltage protection circuits are, in particular, protection circuits working on the principle of “active Zenering”. This principle is likewise presented in the above-mentioned data sheet for the BTS 3410 G and will be explained hereinbelow with reference to FIG. 1.
FIG. 1 shows a circuit arrangement with two semiconductor switches T1, Tn, configured as a MOSFET, each of them having one control terminal 11, 1n as well as first load terminals 21, 2n and second load terminals 31, 3n. Semiconductor switches T1, Tn are actuated via driver circuits DRV1, DRVn depending on input signals IN1, INn fed to the driver circuits DRV1, DRVn. The semiconductor switches T1, Tn and the driver circuits DRV1, DRVn are integrated together in a semiconductor chip, as is indicated in FIG. 1 schematically by the dot-and-dash line indicated as 10.
To protect the semiconductor switches T1, Tn against an overvoltage between their load terminals 21, 31 and 2n, 3n, each of the semiconductor switches T1, Tn has a protection circuit with a diode D1, Dn and a Zener diode Z1, Zn, which are connected between the first load terminals 21, 2n and the control terminals 11, 1n of the semiconductor switches T1, Tn. If, during the operation of this semiconductor switch, an electrical potential at the first load terminal 21, 2n rises to a value lying above the potential on the control terminals 11, 1n by the value of the breakthrough voltage of the Zener diodes Z1, Zn of the protection circuits, the semiconductor switches T1, Tn will be biased into conduction, thereby preventing a further rise in the voltage across their loads and protecting the semiconductor switches T1, Tn against overvoltage.
The drawback in the solution explained by FIG. 1 is that a separate overvoltage protection circuit has to be provided for each semiconductor switch, which requires a not insignificant surface area on the semiconductor chip to produce the protection circuits. This problem becomes worse with increasing number of semiconductor switches being integrated together in a single chip.
Another problem with providing separate overvoltage protection circuits for the at least two semiconductor switches can arise in the situation represented by the dotted line in FIG. 1, when the two semiconductor switches T1, Tn jointly actuate an inductive load L. The load in this case is connected between a terminal for a positive power supply potential V+ and the first load terminals 21, 2n of the two semiconductor switches T1, Tn, while the second load terminals 31, 3n of the semiconductor switches T1, Tn lie at a reference potential GND. Thus, the two semiconductor switches T1, Tn are connected up in parallel for the actuation of the inductive load L. Such a parallel circuit can be required when the current uptake of the load L is higher than the maximum allowable current load of one of the semiconductor switches T1, Tn.
If, in this circuit configuration, the two semiconductor switches T1, Tn are caused to go into conduction, a load current will flow through the inductive load, being shared between the two semiconductor switches T1, Tn. If the two semiconductor switches T1, Tn are then switched off, the voltage on their first load terminals 21, 2n will rise on account of the energy accumulated in the inductive load. If this potential reaches a value where the Zener diodes Z1, Zn conduct a current to the control terminals 11, 1n of the semiconductor switches T1, Tn, the two semiconductor switches T1, Tn will go into conduction to prevent a further rise in the electrical potential at the first load terminals 21, 2n and commutate the inductive load away.
If the Zener diodes Z1, Zn differ in their breakthrough voltage due to manufacturing variations, an operating situation may result in which one of the two semiconductor switches T1, Tn is conducting, while the other is still blocking. The conducting semiconductor switch will then have the entire commutation current of the inductive load flowing through it which can result in an overstress that can lead to destruction of this semiconductor switch. In this connection, it should be noted that, instead of only one Zener diode, it is customary to provide series circuits of several Zener diodes, which somewhat aggravates the problem of manufacturing-caused variations.
An advantage of the present invention is to provide a circuit arrangement with at least two semiconductor switches and an overvoltage protection arrangement in which the overvoltage protection arrangement is economical in space and in which the at least two semiconductor switches are reliably protected against overvoltages.